(Al,Ga,In)N DIODE LASER FABRICATED AT REDUCED TEMPERATURE

ABSTRACT

A method of fabricating an (Al,Ga,In)N laser diode, comprising depositing one or more III-N layers upon a growth substrate at a first temperature, depositing an indium containing laser core at a second temperature upon layers deposited at a first temperature, and performing all subsequent fabrication steps under conditions that inhibit degradation of the laser core, wherein the conditions are a substantially lower temperature than the second temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 ofco-pending and commonly-assigned U.S. Utility patent application Ser.No. 12/476,208, filed on Jun. 1, 2009, by Daniel A. Cohen, Steven P.Denbaars and Shuji Nakamura, entitled “(Al,Ga,In)N DIODE LASERFABRICATED AT REDUCED TEMPERATURE,” attorneys' docket number30794.265-U.S.-U1 (2008-416-2), which application claims the benefitunder 35 U.S.C. Section 119(e) of co-pending and commonly-assigned U.S.Provisional Patent Application Ser. No. 61/057,519, filed on May 30,2008, by Daniel A. Cohen, Steven P. Denbaars and Shuji Nakamura,entitled “(Al,Ga,In)N DIODE LASER FABRICATED AT REDUCED TEMPERATURE,”attorneys' docket number 30794.265-U.S.-P1 (2008-416-1), both whichapplications are incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned U.S. patent applications:

U.S. patent application Ser. No. 11/454,691, filed Jun. 16, 2006, byAkihiko Murai, Christina Ye Chen, Daniel B. Thompson, Lee S. McCarthy,Steven P. DenBaars, Shuji Nakamura, and Umesh K. Mishra, entitled“(Al,Ga,In)N AND ZnO DIRECT WAFER BONDING STRUCTURE FOR OPTOELECTRONICAPPLICATIONS AND ITS FABRICATION METHOD,” attorney's docket number30794.134 -U.S.-U1 (2005-536), now U.S. Pat. No. 7,719,020 issued May18, 2010, which application claims the benefit under 35 U.S.C. Section119(e) of:

U.S. Provisional Application Ser. No. 60/691,710, filed on Jun. 17,2005, by Akihiko Murai, Christina Ye Chen, Lee S. McCarthy, Steven P.DenBaars, Shuji Nakamura, and Umesh K. Mishra, entitled “(Al,Ga,In)N ANDZnO DIRECT WAFER BONDING STRUCTURE FOR OPTOELECTRONIC APPLICATIONS ANDITS FABRICATION METHOD,” attorneys' docket number 30794.134-U.S.-P1(2005-536-1);

U.S. Provisional Application Ser. No. 60/732,319, filed on Nov. 1, 2005,by Akihiko Murai, Christina Ye Chen, Daniel B. Thompson, Lee McCarthy,Steven P. DenBaars, Shuji Nakamura, and Umesh K. Mishra, entitled “(Al,Ga, In)N AND ZnO DIRECT WAFER BONDED STRUCTURE FOR OPTOELECTRONICAPPLICATIONS, AND ITS FABRICATION METHOD,” attorneys' docket number30794.134-U.S.-P2 (2005-536-2); and

U.S. Provisional Application Ser. No. 60/764,881, filed on Feb. 3, 2006,by Akihiko Murai, Christina Ye Chen, Daniel B. Thompson, Lee S.McCarthy, Steven P. DenBaars, Shuji Nakamura, and Umesh K. Mishra,entitled “(Al,Ga,In)N AND ZnO DIRECT WAFER BONDED STRUCTURE FOROPTOELECTRONIC APPLICATIONS AND ITS FABRICATION METHOD,” attorneys'docket number 30794.134-U.S.-P3 (2005-536-3);

all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to diode lasers and methods of fabrication ofdiode lasers.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

The present invention pertains to diode lasers that operate atwavelengths between 470 and 630 nm—a spectral range poorly served by anydiode laser technology. Such lasers would find wide use in scientific,biomedical, sensing, illumination and display applications, supplantingolder technologies in existing markets and enabling new marketsdependent on the unique advantages of diode lasers. These advantagesinclude small size, low cost, high efficiency, and the capability forhigh-speed modulation.

Of particular interest is the green spectral range, approximately495-570 nanometers (nm). When combined with commercially available blueand red diode lasers, a green-emitting diode laser would enable highquality full color projection displays for a very large commercialmarket.

While green-emitting diode lasers based on the II-VI material systemhave been demonstrated, their operating lifetime was so short that theycould not achieve commercial viability, and development of such II-VIlasers has largely been abandoned [5]. Argon ion gas lasers operating at488 nm and 514 nm are widely used for several of the applications listedabove, but are far too large and inefficient for use in mass markets.

Frequency-doubled solid state lasers emitting at 532 nm, pumped byflashlamps or near infrared diode lasers, are also in widespread use forscientific and material processing applications. The smallest versionsof these frequency-doubled diode pumped solid state lasers do manifestmany of the advantages of diode lasers: they are small, lightweight, andare capable of high frequency modulation. Yet compared to diode lasersdirectly emitting the desired wavelength, the frequency-doubled DiodePumped Solid State (DPSS) lasers are inefficient, low-powered, andcostly to manufacture. For example, state-of-the-art miniaturefrequency-doubled DPSS lasers [6] attain 61% coupling efficiency betweenthe pump laser and the doubling crystal, and 58% efficiency in thedoubling crystal. Combined with the 47% electrical-to-optical efficiencyof the pump laser, the overall wall plug efficiency of the doubled DPSSmodule is only 17%, one third that of the original pump laser. Inaddition, the lenses, and doubling crystal, and the careful alignmentneeded during manufacture add significant cost to the frequency doubledDPSS laser, costs not incurred by a direct emitting diode laser.

Diode lasers based on the (Al,Ga,In)As material system have beencommercialized, with direct emission wavelengths from approximately 1000nm down to 630 nm. Light emitting diodes based on (Al,Ga,In)P have beencommercialized at wavelengths down to the orange range, 570-590 nm, butlaser operation has not been achieved in that material system at thatwavelength range. Diode lasers based on the (Al,Ga,In)N material systemhave been commercialized with direct emitting wavelengths from 370 nm upto 470 nm, and record operation up to 510 nm [7], and while green LEDshave been made in (Al,Ga,In)N, laser operation in the green spectralrange remains elusive. To obtain green emission, the indium molefraction of the active region must be increased to approximately 25%,compared to 10-15% used in violet emitting lasers and 20% in true bluelasers. It is widely believed that the limitation in the (Al,Ga,In)Nsystem is that the active region material quality degrades duringsubsequent crystal growth of the upper waveguide and electrical contactlayers used in conventional diode lasers, and that this degradationworsens as the indium content, temperature, and growth time increase.

One solution is to develop low-temperature crystal growth techniquessuch as plasma-assisted molecular beam epitaxy (PAMBE) or ammoniamolecular beam epitaxy (NH₃-MBE), which can use growth temperaturessignificantly lower than those used in the more common growth technique,metalorganic chemical vapor deposition (MOCVD). Indeed, violet-emitting(Al,Ga,In)N diode lasers grown entirely by PAMBE have been demonstrated[8]. It is not yet established that molecular beam epitaxy (MBE) growthwill be able to produce green-emitting lasers, for the low growthpressure compared to MOCVD does not favor high indium incorporation orhigh crystal quality.

Other solutions include a number of different techniques as describedbelow:

Kim [9] has grown single crystal epitaxial zinc oxide (ZnO) layers on(Al,Ga,In)N LEDs using a low temperature hydrothermal method, for thepurpose of improved light extraction from LEDs. ZnO lateral epitaxialovergrowth (LEO) onto (Al,Ga,In)N Light Emitting Diodes (LEDs) was alsodescribed [10]. The ZnO formed a transparent electrical contact, with nowaveguiding function, and use with diode lasers was not discussed.

Sasaoka [1] has described a method to form (Al,Ga,In)N ridge waveguidesby high temperature LEO of (Al,Ga,In)N layers through an opening in anAlN mask, for the purpose of improving the performance of lasersemitting violet or blue light. The regrown waveguides performed bothelectrical and optical functions, but were grown at temperatures thatwould degrade (Al,Ga,In)N active regions intended for green emission.

Fang [2] has described a technique to bond semiconductor active regionsformed from (Al,Ga,In)As and (Al,Ga,In)(As,P), grown on GaAs or InPsubstrates respectively, to ridge waveguides formed insilicon-on-insulator substrates, specifically for integration of III-Voptoelectronic components with silicon electronics. The silicon ridgewaveguides were electrically passive, unlike the conducting waveguidesproposed in this application. They have demonstrated workingelectrically-pumped lasers, photodetectors, and amplifiers operating atnear infrared wavelengths. They did not claim any applicability to othermaterial systems.

Sink [3] has described a technique to bond (Al,Ga,In)N laser structuresto cubic substrates such as GaAs or InP, with subsequent removal of thegrowth substrate, specifically to facilitate the cleaving of highquality laser facets. The cleavage substrate performed no optical role.

Murai [4] has described a technique to bond bulk ZnO to (Al,Ga,In)N LEDstructures for the purpose of enhanced light extraction from LEDs. Thebonded ZnO performed both an electrical and optical role (See also, U.S.patent application Ser. No. 11/454,691, filed on Jun. 16, 2006, byAkihiko Murai et. al, entitled “(Al,Ga,In)N AND ZnO DIRECT WAFER BONDINGSTRUCTURE FOR OPTOELECTRONIC APPLICATIONS AND ITS FABRICATION METHOD,”which application is incorporated by reference herein.)

Margalith [15] has described transparent electrical contacts to(Al,Ga,In)N, formed by sputter deposition of indium tin oxide (ITO) ortitanium nitride, for the purpose of enhanced light extraction from LEDsand for low loss electrical contacts to vertical cavity surface emittinglasers. Surface emitting laser operation was not achieved, and nowaveguiding function or other use with in-plane lasers was proposed.

The key invention described here is to use unconventional designs andfabrication methods to eliminate the prolonged growth at hightemperature after the laser active region is formed, while retaining thebenefits of MOCVD growth of the active region

SUMMARY OF THE INVENTION

Throughout this disclosure, the present invention refers to the use ofGroup III-Nitride materials, which may be referred to as III-N,III-Nitride, AlGaInN, or (Al,Ga,In)N, with slightly differentconnotations. It is noted, for example, that AlGaInN encompasses thebinary and ternary alloys GaN, AlGaN, and InGaN as well.

Throughout this disclosure, the present invention refers to a III-Nactive region and to a III-N laser core. It is common in the art to usethe term active region to refer to the layers in which light isgenerated, commonly one or more quantum wells. In this disclosure, theterm laser core refers to that part of the laser in which light isgenerated and largely confined, and comprises the active region layers,the III-N layers surrounding the active layers that serve to confinecarriers, commonly called barrier layers, the III-N layers surroundingthe outermost barrier layers that serve to inject carriers into thebarrier and active region layers and may also serve as waveguide corelayers, and III-N layers that serve to further confine carriers to theactive region, commonly referred to as electron blocking layers or holeblocking layers.

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesan (Al,Ga,In)N based diode laser comprising an (Al,Ga,In)N laser corefor generating and confining light having a laser wavelength; and atleast one transparent and electrically conducting layer deposited on thelaser core to provide electrical contact to the laser core, wherein thetransparent and electrically conducting layer (i) has a refractive indexlower than a refractive index of the laser core, (ii) is deposited by amethod other than crystal growth at a temperature above 550° C., (iii)has a bulk resistivity below 10 Ohm-centimeters (Ohm-cm) and is capableof making an electrical contact to the laser core with a specificcontact resistance below 0.01 Ohm-cm², and (iv) has an opticalabsorption coefficient below 2000 cm⁻¹ measured at the laser wavelength.

The transparent and electrically conducting layer typically has aconductivity sufficiently high to provide ohmic electrical contact tothe active layer, so that electrons or holes in the transparent andelectrically conducting layer may be driven by an external voltage intothe active region where they recombine with carriers of the oppositecharge and injected from the opposite side of the active region, togenerate light.

The transparent and electrically conducting layer may be transparent tothe light generated in the laser core, and may serve as a waveguidecladding layer that confines the light generated in the laser core. Thetransparent and electrically conducting layer may be amorphous orcrystalline, for example. Materials for the transparent and electricallyconducting layer comprise materials different from a III-Nitridematerial, such as tin-doped indium oxide (ITO), ZnO, indium oxide, tinoxide, gallium oxide, magnesium oxide, cadmium oxide, other metaloxides, alloys of these compounds, as well as III-N materials.

The transparent and electrically conducting layer may be patterned in aplane of the transparent and electrically conducting layer to form a ribor ridge, to serve as a lateral waveguide core. A wafer bond may bebetween the transparent and electrically conducting layer and the lasercore.

The lower injection layer of the laser core (and therefore the lasercore) may be deposited on a transparent layer of lower refractive index,such that the transparent layer of lower refractive index serves as atransverse optical waveguide cladding layer. The transparent andelectrically conducting layer, serving as an upper cladding layer thatimproves confinement of light to the laser core, can be a growth on theIII-N upper injection layer of the laser core. The laser core may be ona lower cladding layer, and the effective refractive index of the lasercore is typically higher than that of the lower cladding layer, andhigher than that of the upper cladding layer, so that the laser corefunctions as a transverse optical waveguide core. The laser core mayalso be patterned in a plane of the laser core to form a rib or ridge,to serve as a lateral waveguide core.

In one example, the active region is at least one indium-containingIII-Nitride layer that is grown on a III-Nitride lower injection layer,wherein the III-nitride lower injection layer serves to inject eitherelectrons or holes into the active region; multiple active region layersmay be employed, separated by III-N layers (e.g. adjacent to each sideof the active layer) serving to confine the electrons and holes in theactive layers in which light is generated; at least one III-Nitrideupper injection layer is grown on the active region, serving as an upperinjection layer for carriers of a type not provided by the lowerinjection layer; embedded in the upper injection layer may be a III-Nblocking layer to limit escape of carriers from the active region; and atransparent and electrically conducting layer on the upper injectionlayer provides electrical contact to the upper injection layer andserves as an upper waveguide cladding layer. In this disclosure, thestructure between the lower cladding layer and the upper cladding layeris referred to as the laser core.

A further example comprises at least one first III-Nitride blockinglayer between the active region and the lower injection layer and/or atleast one second III-Nitride blocking layer between the active regionand the upper injection layer, wherein (i) the first III-Nitrideblocking layer serves to prevent injection of first minority carriersinto the lower injection layer and the first minority carriers arecarriers of the type not provided by the lower injection layer, and (ii)the second III-Nitride blocking layer serves to prevent injection ofsecond minority carriers into the upper injection layer and the secondminority carriers are carriers of a type not provided by the upperinjection layer.

The present invention further discloses a method of fabricating an(Al,Ga,In)N based diode laser comprising depositing at least onetransparent and electrically conducting layer on a laser core of thediode laser, wherein a refractive index of the transparent andelectrically conducting layer is lower than an effective refractiveindex of the laser core, and the transparent and electrically conductinglayer provides electrical contact to the laser core.

The method may further comprise depositing (by e.g., MOCVD) an indiumcontaining (Al,Ga,In)N laser core.

The transparent and electrically conducting layer may comprise acrystalline transparent conductive oxide layer grown from an aqueoussolution, a combination of a crystalline transparent conductive oxidelayer grown from an aqueous solution and an amorphous or polycrystallinetransparent conducting oxide layer deposited by physical vapordeposition, an amorphous or polycrystalline transparent conducting oxideformed by physical vapor deposition, a crystalline transparentconductive oxide layer wafer bonded to the laser core, or crystalline(Al,Ga,In)N layers grown on a sacrificial substrate and wafer bonded tothe laser core.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic representation of a conventional (Al,Ga,In)N ridgewaveguide laser, wherein layers are not drawn to scale, and bufferlayers and ridge dielectric isolation are not shown.

FIG. 2 is a schematic representation of a ridge waveguide laser usingcrystalline ZnO as the upper cladding and contact layer, wherein layersare not drawn to scale and buffer layers and ridge dielectric isolationare not shown.

FIG. 3( a) is a simulation of a fundamental optical mode profile in aZnO-clad ridge waveguide laser.

FIG. 3( b) is a schematic representation of an indium tin oxide cladInGaN/GaN MQW laser, wherein replacement of the conventional p-AlGaNupper cladding with e-beam evaporated ITO reduces MQW degradation duringgrowth of subsequent layers, and improves mode overlap with the activeregion.

FIGS. 4( a)-4(e) are schematic representations of a wafer bondingapproach, wherein buffer layers are not shown, after wafer bonding thesacrificial substrate is removed, and a ridge waveguide is formed,followed by anode and cathode contact deposition.

FIG. 5 is a schematic representation of a ridge waveguide laser usingamorphous indium tin oxide (ITO) as the upper cladding and contactlayer, wherein buffer layers and ridge dielectric isolation are notshown.

FIG. 6 is a flowchart illustrating a method of the present invention.

FIG. 7 shows the spectrum from an electrically pumped, ITO clad InGaNMQW laser, plotting intensity (counts per second, cps) as a function ofwavelength of the light (nanometers, nm) wherein resonator modes riseabove the spontaneous emission, indicating the device is near lasingthreshold.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

A tremendous market exists for diode lasers that operate in the greenrange, 495-570 nm, but this range has only been achieved withshort-lived diode lasers fabricated in the II-VI material system or byfrequency-doubling solid state lasers optically pumped by(Al,Ga,In)As-based diode lasers operating in the near infrared range.Neither of these previously demonstrated methods provide adequate poweror efficiency for important biomedical, sensing, illumination anddisplay applications. While light emitting diodes (LEDs) that emit atwavelengths spanning the 250-560 nm spectral range have been fabricatedfrom the (Al,Ga,In)N material system, diode lasers fabricated from thismaterial system have been limited to operation below 510 nm. It iswidely believed that the limitation is due to degradation of the lasercore during the prolonged high temperature crystal growth needed tocomplete the optical waveguide and electrical contact layers grown afterthe laser core.

The key invention described here is the reduction or elimination of thehigh temperature process steps after laser core growth, to allowoptimization of the laser core for longer wavelength operation. Threerelated approaches are described that replace the traditionalepitaxially-grown (Al,Ga,In)N upper waveguide and electrical contactlayers with layers formed by lower temperature processes. One approachuses a crystalline transparent conducting oxide such as ZnO, growndirectly on the laser wafer or grown separately and transferred to thelaser wafer in a moderate-temperature bonding step. Another approachuses crystalline (Al,Ga,In)N layers grown on a separate substrate andthen transferred to the laser wafer with a bonding and substrate removaltechnique. A third approach uses amorphous or polycrystallinetransparent conducting oxides deposited directly onto the laser waferwith low temperature processes. In all cases, the time and temperatureto which the laser laser core is exposed is significantly reduced,thereby reducing degradation of the laser core.

Two additional advantages are expected in the embodiments based ontransparent conducting oxides. The high index contrast between(Al,Ga,In)N alloys and transparent conducting oxides and the potentialreduction of the p-type GaN needed compared to conventional laserdesigns, affords significant enhancement of the optical confinement inthe laser core and reduction of optical loss. Also, the high electricalconductivity of transparent conducting oxides and the reduced claddingthickness needed compared to conventional designs allows reduction ofthe operating voltage.

Technical Description

A conventional (Al,Ga,In)N diode laser 100 is shown schematically inFIG. 1. The device comprises a substrate 102, typically GaN, n-type GaN(n-GaN), or sapphire, followed by a GaN buffer layer (not shown), ann-type (Al,Ga)N cladding layer 104 such as an n-type AlGaN (n-AlGaN)lower waveguide cladding layer, and an n-type GaN lower waveguide layeror core 106. An (Al,Ga,In)N active region 108 follows, which istypically a single quantum well or multiple quantum wells (MQW) withmultiple barriers (e.g., InGaN/GaN MQW). The active region 108 isfollowed by an (Al,Ga)N electron blocking layer 110 such as a p-typeAlGaN (p-AlGaN) electron blocking layer, a p-type GaN (p-GaN) waveguidelayer 112 (e.g. p-GaN upper waveguide core), a p-type (Al,Ga)N upperwaveguide cladding layer (e.g., p-AlGaN) 114, a thin p⁺-type contactlayer such as p⁺-type GaN (p⁺-GaN) 116, and metal contact layers (metalanode 118 and metal cathode 120). The n-type layers 104, 106 below theactive region are typically grown at temperatures above 1100 degreesCelsius (° C.), whereas the active region 108 is typically grown between850-950° C., and the layers 110, 112, 114 and 116 above the activeregion 108 are grown at 1000-1100° C.

The layers 110, 112, 114 and 116 above the active region 108 must have acombined thickness 122 sufficient to prevent the optical field thatpropagates in the waveguide layers 112, 114 from penetrating to themetal contact 118, for this would lead to unacceptably high opticalabsorption loss. For this reason, these upper layers 110, 112, 114 and116 typically have a thickness 122 in a range of 0.75-1.5 micrometers.

It is during the high temperature growth of these upper waveguide 112,cladding 114 and contact layers 116 that the (Al,Ga,In)N active region108 is thought to degrade. The key innovation of this invention is toreplace all or part of these layers 112, 114 and 116 grown at hightemperature after the active region 108 growth with layers that performequivalent functions but that are formed without exposing the activeregion 108 to conditions that promote degradation. Three relatedapproaches are described, each with possible variations.

In one approach, the upper cladding 114 and contact layers 116 arereplaced by a crystalline transparent conductor, such as ZnO, depositedon the laser wafer by a low temperature process, such as growth from anaqueous solution. In another approach, transparent conducting layers of(Al,Ga,In)N alloys are grown at high temperature on a separatesubstrate, and then transferred to the laser wafer by low or moderatetemperature bonding and substrate removal steps to complete thewaveguide and electrical contact layers. In a third approach, the uppercladding and contact layers are formed from an amorphous orpolycrystalline transparent conductor, such as ITO, deposited at lowsubstrate temperature by evaporation or sputtering, possibly followed bya moderate temperature annealing step. In all three approaches, theactive region is grown under optimum conditions, and not later exposedto environments that lead to degradation.

FIG. 2 is a cross-sectional schematic of an (Al,Ga,In)N based diodelaser 200 comprising an (Al,Ga,In)N laser core (comprising an activeregion 202) for generating and confining light having a laserwavelength; and at least one transparent and electrically conductinglayer 204 (different from a III-Nitride) adjacent (or deposited, oroverlaid on) the laser core to provide electrical contact to the lasercore, wherein the transparent and electrically conducting layer 204 hasa refractive index lower than an effective refractive index of the lasercore. FIG. 2 is the preferred implementation of the first approach, inwhich the AlGaN upper cladding 114 and p-contact layers 116 are replacedby a layer of crystalline ZnO 204 (e.g., ZnO upper waveguide claddinglayer), the p-GaN upper waveguide core is patterned to form a ridge 206waveguide to serve as a lateral waveguide core, isolated with alower-index dielectric such as silicon dioxide (SiO₂) (not shown), andthen contacted with a metal layer 208 (metal anode). The ZnO transparentand electrically conducting layer 204, serving as an upper waveguidecladding layer, is on a III-N upper injection layer 210 (e.g., p-GaNupper waveguide core), and therefore also on the laser core.

The active region 202 is between the upper injection layer 210 and alower injection layer 212. The indium-containing III-Nitride activeregion 202 (e.g., InGaN—GaN MQW) is on the III-Nitride lower injectionlayer 212 (e.g., n-GaN lower waveguide core), wherein the III-Nitridelower injection layer 212 serves to inject electrons into the activeregion 202. The III-Nitride upper injection layer 210 on the activeregion 202 serves as an upper injection layer for carriers (e.g., holes)of a type (e.g. p-type) not provided by the lower injection layer 212.The laser core comprises the active region 202 on layer 212, and layer210 on layer 202. The device's 200 layers may be grown, for example.

The effective refractive index of the laser core is higher than that ofthe lower cladding layer 214, and higher than that of the upper claddinglayer 204, so that the laser core functions as a transverse opticalwaveguide core.

The upper injection layer 210, and therefore the laser core, ispatterned in a plane of the layer 210, to form a rib or ridge 206, toserve as a lateral waveguide core.

Also shown are the n-AlGaN lower waveguide cladding 214, n-GaN substrate216, and metal cathode 218.

ZnO has an index of refraction of approximately 2.0 in the greenspectral region, appropriate for both transverse and lateral confinementof the optical mode 300 as simulated in FIG. 3( a), in which contours ofequal optical intensity are shown for a single transverse optical mode.Also shown in FIG. 3( a) is the SiO₂ 302, p-GaN upper waveguide 210,active region 202, n-GaN lower waveguide 212, and n-GaN substrate 216.Optionally, as noted above, one or more III-N layers 210 may bepatterned to form a shallow rib 206 to aid in lateral opticalconfinement. Single crystal ZnO has been shown to have low optical lossin the visible spectrum and low electrical resistance, able to makereasonably good electrical contact to p-type GaN, and that theseproperties may be obtained with ZnO grown on GaN from a low temperatureaqueous bath [9].

FIG. 3( b) is a schematic representation of an ITO 304 clad InGaN/GaNMQW 202 laser, having the structure of FIG. 2 (but using ITO 304 insteadof ZnO 204). Also shown are lower and upper III-N separate confinementheterostructure layers 220 and 222, which further confine the opticalmode to the laser core, upper blocking layer 224, and anode contact 208.

ZnO may be patterned by both wet and dry etching, or alternatively grownthrough a mask using an aqueous growth version of LEO [10]. The mask maybe a dielectric such as SiO₂ and left in place after the LEO step, or itmay be a soluble or etchable material such as photoresist and removedafter the LEO step, leaving an air gap.

The AlGaN electron-blocking layer 110 shown in FIG. 1 is not universallyused, and it has not been established that it is needed in greenemitting diode lasers. It may prove advantageous to omit it. It has alsobeen shown that when enough InGaN material is grown in the active region108 or a separate confinement heterostructure, it is possible toeliminate the AlGaN cladding layer above 114 or below 104 the activeregion 108, or entirely [11]. In FIG. 2, both the electron-blockinglayer 110 and the upper AlGaN cladding layer 114 have been omitted. Thefundamental optical mode profile 300 in this device is shown in FIG. 3.The mode 300 is well confined in both transverse and lateral directionsand provides a good overlap with the active region 202 for high modalgain. The high index contrast between GaN and ZnO, compared to GaN andAlGaN, leads to tighter confinement of the optical mode and reduced modeoverlap with lossy p-type layers.

The second basic approach, based on wafer bonding and substrate removal,results in a device similar to the conventional device shown in FIG. 1.The fabrication process is shown schematically in FIGS. 4( a)-(e).Conventional crystal growth methods are used to grow part of the laserstructure 400 through the active region 402 and stopping with a thinp-GaN layer 404, as shown in FIG. 4( a). For example, the structure 400may comprise n-AlGaN lower cladding layer 406 on n-GaN substrate 408,n-GaN lower waveguide core 410 on the n-AlGaN lower cladding layer 406,InGaN-GaN active region 402 on the n-GaN lower waveguide core 410, andhalf of a p-GaN waveguide core 404 on the active region 402. On a secondsacrificial substrate 412, as shown in FIG. 4( b), a p-GaN 414 (theother half of a p-GaN waveguide core) and optional p-AlGaN layer 416(e.g., p-AlGaN electron blocking layer) are grown, along with a p⁺-GaNcontact layer 418, grown in reverse order to their position in the finaldevice. A p-AlGaN upper cladding layer 420 and InGaN separation layer422 may also be grown in the wafer 424. The two substrates 408 and 412are then aligned (as shown in FIG. 4( c)), clamped together, and heatedat a temperature adequate to cause bonding of the two wafers 400 and424, but low enough to avoid degradation, likely in the range of600-750° C. (as shown in FIG. 4( d)), thereby forming a wafer bond 426between layers 404 and 414. After bonding, the sacrificial substrate 412is removed by a combination of laser-assisted liftoff, physicalpolishing and chemical or photoelectrochemical etching, leaving amultilayer structure similar to the conventional single-growthstructure, ready for completion by standard fabrication methods, asshown in FIG. 4( e). The p-GaN waveguide cores 404, 414 are bonded toform a single p-GaN waveguide core 428.

Thus, FIG. 4( e) illustrates an indium-containing III-N active region402 grown on other III-N layers, including a III-N lower waveguidecladding layer 406, and a III-N lower waveguide layer 410, wherein theother III-N layers 406, 410 are grown on a first growth substrate 408,the active region 402 comprising (1) at least one indium-containingactive layer 430 in which light is generated (e.g. InGaN quantum well);and (2) III-N confinement layers 432 (e.g., GaN barriers) adjacent toeach side of the active layer, serving to confine carriers (electronsand holes) in the active layer 430; (b) an (Al,Ga,In)N contact layer 418grown on a sacrificial substrate 412 and (Al,Ga,In)N separation layer422; (c) an (Al,Ga,In)N upper waveguide cladding layer 420 and(Al,Ga,In)N, grown on the contact layer 418, acting as an upperwaveguide cladding layer; and (d) a wafer bond between one half of the(Al,Ga,In)N upper waveguide core, layer 414 ,and the other half of upperwaveguide core, layer 404, forming upper waveguide core layer 428 suchthat the lower waveguide core 410 layer and the upper waveguide corelayer 428 bound the active region 402.

The position of the blocking layer 416 varies, and in such awafer-bonded device it may also be on the wafer 400 with the activeregion 402.

It is generally found that superior electrical performance is obtainedby bonding similarly-doped layers, either n-type to n-type or p-type top-type, to avoid minority carrier loss due to defects at the bondinterface. Thus, it would be necessary to grow at least a thin p-typeIII-N epitaxial layer 404 above the active region 402 in theimplementation shown in FIGS. 4( a)-(e).

A possible variation of this approach is to use a commercially availablebulk ZnO wafer instead of the (Al,Ga,In)N-sacrificial substratecombination: the ZnO may be bonded to the laser wafer at temperaturesaround 600° C., then mechanically or chemechanically thinned andpolished to the desired thickness, followed by ridge formation andcontact metal deposition [12]. It has been found that using a very thinmetallic layer between the p-GaN and the ZnO improves the electricalcontact, although the additional optical loss due to absorption in themetal must be considered.

In the third approach, the epitaxial upper waveguide 114 and contactlayers 116 are replaced with an amorphous transparent conducting oxidesuch as evaporated or sputtered ITO. The optical absorption loss ofamorphous ITO is 350-1000 cm⁻¹ at 500 nm, high compared to the 10 cm⁻¹of single crystal ZnO, but still acceptable since the high indexcontrast between ITO and GaN results in a small optical overlap with theITO. A variation utilizes a combination of crystalline and amorphoustransparent conducting oxides, such as a layer of aqueous-grown ZnO,100-200 nm thick, followed by a layer of evaporated or sputteredamorphous ITO. In this case, the benefits of crystalline ZnO areobtained while avoiding the difficulties in fabricating thickercrystalline ZnO layers.

FIG. 5 is a cross-sectional schematic of an example of the thirdapproach, an (Al,Ga,In)N based diode laser 500 comprising an (Al,Ga,In)Nactive region 502 for emitting light; and ITO as the at least onetransparent and electrically conducting layer 504 deposited on theactive region 502 to provide electrical contact to the active layer 502,wherein the transparent and electrically conducting layer has arefractive index lower than a refractive index of the active region 502.The indium-containing III-Nitride active region 502 (e.g., InGaN—GaNMQW) is grown on a III-Nitride lower injection layer 506 (e.g., p-GaN)wherein the lower injection layer 506 serves to inject holes into theactive region 502. A p-AlGaN electron blocking layer 508 is between theactive region 502 and the lower injection layer 506, the p-GaN 506 is onan AlGaN lower waveguide cladding layer 510, the waveguide claddinglayer 510 is on a GaN substrate 512, a metal cathode 514 is on the ITO504, and metal anodes 516 are on the p-GaN 506. The laser core comprisesthe active region 502, injection layer 506, and at least one first III-Nblocking layer 508 (electron blocking layer). The layer 508 is betweenthe active region 502 and the lower injection layer 506, and serves toprevent injection of first minority carriers into the lower injectionlayer 506, wherein the first minority carriers are carriers of the typenot provided by the lower injection layer 506. In this case, theblocking layer 508 prevents injection of electrons into a hole injectionlayer 506.

Alternatively, or in addition, at least one second III-N blocking layermay be provided, between the active region and an upper injection layer,that serves to prevent injection of second minority carriers into theupper injection layer, wherein the second minority carriers are carriersof the type not provided by the upper injection layer. For example, thesecond III-N blocking layer may prevent injection of holes into theupper injection layer if the upper injection layer is an electroninjection layer.

Ohmic contacts have been achieved to both p-type [13] and n-type [14]GaN. To take advantage of this, the GaN p-contact and hole injectionlayer 506 may be moved below the active region 502, along with theoptional AlGaN electron blocking layer 508 as shown schematically inFIG. 5. This eliminates the need to grow p-GaN 112 and p-AlGaN 110 abovethe active layer 108, further reducing the likelihood of active region108 degradation. It is generally found that the surface of p-GaNroughens as the thickness of the layer increases, which might interferewith good growth of a quantum well active region. For this reason, it isadvantageous to keep the p-GaN layer as thin as needed for adequatecurrent transport from the anode contact to the active region: athickness of approximately 0.5 micron should provide adequateconductivity without significant roughening.

Which of these various approaches will be the most successful depends ona balance between electrical conductivity and optical loss, the extentto which each approach allows optimization and retention of the activeregion quality, and on fabrication and reliability issues. Based on thebest demonstrated contact resistance between ITO and p-GaN and on thebest measured optical transparency of ITO, and on the known fabricationtechnology and reliability of ITO contact to III-N light emittingdevices, the preferred approach is that shown in FIG. 3( b), with a thinp-GaN hole injection and waveguide core layer 210 grown above the activeregion 202, followed by an ITO transparent and electrically conductinglayer 304. The preferred fabrication technique for the ITO is byphysical vapor deposition.

Alternative transparent conductive oxides include, but are not limitedto, indium oxide, tin oxide, zinc oxide, gallium oxide, magnesium oxide,cadmium oxide, and various alloys of these compounds, all with a varietyof doping variants. Generally, these materials may also be deposited byevaporation or sputtering techniques. Transparent conductors other thanoxides also exist and may be useful: TiN/ITO contacts to p-GaN have beendemonstrated [15]. The choice again depends on the balance betweenelectrical conductivity and optical loss, and on fabrication andreliability issues. The transparent and electrically conducting layertypically have an optical absorption coefficient below 2000 cm⁻¹,measured at the laser wavelength, and a bulk resistivity below 10Ohm-cm, and be capable of making an electrical contact to the laser corewith a specific contact resistance below 0.01 Ohm-cm². However, othervalues for the bulk resistivity, specific contact resistance, andabsorption coefficient are also possible.

FIG. 6 illustrates a method of fabricating an (Al,Ga,In)N laser diode.The method comprises one or more of the following steps:

Block 600 represents depositing one or more III-N layers upon a growthsubstrate.

Block 602 represents depositing an indium containing active region. Theindium containing active region layers may be deposited upon the III-Nlayers at a first temperature. The indium containing active layertypically has a first side and a second side. For example, an indiumcontaining (Al,Ga,In)N active region may be deposited at a firsttemperature on one or more (Al,Ga,In)N first waveguide core or claddinglayers, wherein a first waveguide core or cladding layer is grown on adevice substrate and the device substrate is on the first side of theactive region. The indium containing (Al,Ga,In)N active region may bedeposited by MOCVD, however, other deposition methods may be used.

Block 604 represents performing subsequent fabrication steps. Forexample, all of the subsequent fabrication steps may be at a secondtemperature that is sufficiently or substantially lower than the firsttemperature of Block 602 (e.g., below 550° C.), to inhibit degradationof the active layer. The subsequent fabrication steps may includedepositing subsequent layers on the second side of the active region,opposite the first side, at a second temperature that is lower than thefirst temperature at which the indium containing active region isdeposited in Block 602, so that the active region is not degraded,wherein the subsequent layers comprise one or more of the followinglayers: at least one second waveguide core layer, at least one secondwaveguide cladding layer, at least one carrier injection layer, at leastone carrier blocking layer, and at least one electrical contact layer.

Examples of the second waveguide cladding layer include, but are notlimited to, (1) a crystalline transparent conductive oxide layer grownfrom an aqueous solution, (2) a combination of a crystalline transparentconductive oxide layer grown from an aqueous solution and an amorphousor polycrystalline transparent conducting oxide layer deposited byphysical vapor deposition, (3) an amorphous or polycrystallinetransparent conducting oxide formed by physical vapor deposition, or (4)a crystalline transparent conductive oxide layer wafer bonded to thelaser core (e.g., to the active region or second waveguide core layer).

Block 606 represents the end result, a device.

SUMMARY

To summarize, this invention describes several related designs andfabrication methods for (Al,Ga,In)N-based diode lasers intendedprimarily for operation at wavelengths between 495-570 nm. Theadvantages over other lasers that may operate in this spectral rangeinclude small size, low weight, low cost, high efficiency, highreliability, and the ability to modulate at high frequency. The expectedperformance improvements over the existing art should be dramatic, sothat new classes of products and applications will be enabled. Theprimary innovations include:

-   -   Reduced exposure of the active region to conditions leading to        degradation, in particular, reduced exposure to elevated        temperature after the active region has been grown.    -   Termination of the high temperature epitaxial crystal growth        before the conventional upper waveguide cladding and contact        layers.    -   Improved optical mode confinement through use of materials with        high refractive index contrast.    -   Use of ZnO grown from an aqueous bath onto GaN, as waveguide        cladding and electrical contact layers.    -   Use of bulk-grown ZnO bonded onto GaN as laser waveguide and        electrical contact layers.    -   Use of (Al,Ga,In)N layers, grown on a sacrificial substrate and        transferred to an incomplete laser structure on another        substrate, as waveguide cladding and electrical contact layers.    -   Use of amorphous or polycrystalline transparent conducting        oxides as waveguide cladding and electrical contact layers.    -   Use of a p-type epitaxial layer or layers grown beneath the        laser active region.

FIGS. 2, 3, 4(a)-(e), 5, and 6 illustrate embodiments of a laser diodethat can emit green light having a wavelength longer than 515 nm, andFIG. 7 shows the spectrum of light emitted from a laser similar to thestructure of FIG. 2, with an ITO upper waveguide cladding instead ofZnO. The indium composition of the indium containing III-nitride activeregion are such that the active layer emits the light having awavelength longer than 515 nm (the indium composition controls the sizeof the active layer's bandgap, which in turn determines the wavelengthof light that is emitted). For example, the indium composition can be atleast 25% (e.g. the active layer can be In_(0.25)Ga_(0.75)N, forexample). The indium composition and crystal quality are at least ashigh as compared to an indium composition and crystal quality of theactive region prior to bonding or growth of (i.e., without) one or morelaser cladding, laser waveguide, or contact layers on the active layer.

Possible Modifications and Variations

The present invention envisages the use of transparent conducting oxidewaveguide cladding and contact layers integrated with the hightemperature epitaxial growth, p-type electrical transport layers grownbeneath laser active regions, wafer-bonded waveguide and electricaltransport layers transferred onto the active regions, and laser designsmust be optimized with regard to performance requirements.

Throughout the disclosure, a transparent conductor may be used insteadof a transparent conducting oxide, to include materials that are neitheroxides nor III-nitrides, such as TiN. The transparent and electricallyconducting layer can be transparent to the light emitted by the activeregion, crystalline, or amorphous, for example. The transparent andelectrically conducting layer is typically a waveguide cladding layerthat aids confinement of the light generated in the active layer,thereby improving confinement of the light to the laser core.

The transparent and electrically conducting layer may be deposited by avariety of methods, for example, deposited or grown (typically by amethod other than crystal growth at a temperature above 550° C.), orwafer bonded (so that the wafer bond is typically between thetransparent and electrically conducting layer and the laser core). Thewafer bond may be in different locations between the transparent andelectrically conducting layer and the active region or the upperinjection layer. For example, the transparent and electricallyconducting layer may interface the active region or upper injectionlayer directly, or there may be one or more additional layers betweenthe active region and the transparent and electrically conducting layer.

While the primary utility of this invention has been identified aslasers operating between 490-570 nm, some or all of the innovationsdescribed here will be of use for diode lasers operating at otherwavelengths, and it is not implied that the scope of this invention islimited to any specific wavelength range. Many variations of theseinnovations exist and may be used in different combinations, and it isnot implied that the scope of this invention is limited to the specificembodiments described here.

Advantages and Improvements

In general, diode lasers are significantly smaller, cheaper, and moreefficient than any other type of laser, and may be easily modulated athigh frequency. For example, miniature frequency doubled diode pumpedsolid state lasers are limited in power to approximately 100 milliwatts(mW), and reach only 17% wall plug efficiency, and are costly tomanufacture due to the need to carefully align a diode pump laser andnonlinear crystal for frequency doubling. In comparison, a diode laseremitting directly in the blue or green range is expected to achieve 50%efficiency or better, provide optical power of hundreds of mW, and besimple to manufacture and package. For these reasons diode lasers havehistorically supplanted other laser types in many markets, once thediode lasers become available.

Currently, lasers emitting in the 485-570 nm spectral range includeargon ion, helium neon, and frequency doubled solid state lasers. Theselasers are used primarily for scientific research, biomedical screeningand drug development, and material processing and are good candidatesfor replacement by diode lasers. More importantly, green emitting diodelasers could be combined with existing blue and red emitting diodelasers to enable mass-produced high quality full color projectiondisplays for consumer use. Such a mass market is of interest to manymanufacturers of electronic components and systems. The presentinvention could be used to manufacture diode lasers for incorporationinto a variety of mass-produced consumer, industrial, medical,scientific, and military products.

Advantages over the current art include higher material gain and accessto longer wavelength operation due to superior crystal quality, andhigher modal gain due to higher refractive index contrast.

REFERENCES

The following references are incorporated by reference herein:

[1] Sasaoka, C., Physica Status Solidi (a), 203:1824-1828 (2006).

[2] Fang, A., Optics Express, 14:9203-9210 (2006); see also Bowers, J.E., U.S. Patent Application 20070170417.

[3] Sink, R. K., Applied Physics Letters, 68:2147-2149 (1996); see alsoBowers, J. E., U.S. Pat. No. 5,985,687.

[4] Murai, A., Applied Physics Letters, 89:171116 (2006).

[5] Okuyama, H., IEICE Transactions on Electronics, E83C:536-545 (2000).

[6] Nguyen, H K, IEEE Photonics Technology Letts., 18:682-685 (2006).

[7] Miyoshi, T., Applied Physics Express, 2:062201 (2009).

[8] Skierbiszcewski, C., Acta Physica Polonica A, 110: 345-351 (2006).

[9] Kim, J. H., Advanced Functional Materials, 17:463-471 (2007).

[10] Andeen, D., Advanced Functional Materials, 16:799-804 (2006).

[11] Feezell, D., Japanese Journal of Applied Physics, 13:L284-L286(2007).

[12] Murai, A., Applied Physics Letters, 89:17116 (2006).

[13] Yao, Y., Displays, 28:129-132 (2007).

[14] Hwang, J. D., Microelectronic Engineering, 77:71-75 (2005).

[15] Margalith, T., Proceedings of the SPIE, 3944 (1-2):10-21 (2000).

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A method of fabricating an (Al,Ga,In)N laserdiode, comprising fabricating an indium containing laser core; andfabricating subsequent layers on the laser core, in a manner thatinhibits degradation of the laser core, wherein the subsequent layersfabricated on the laser core comprise a crystalline, polycrystalline oramorphous transparent conducting oxide grown or deposited on the lasercore.
 2. The method of claim 1, wherein the subsequent layers compriseone or more of: at least one waveguide core layer, at least onewaveguide cladding layer, at least one carrier injection layer, at leastone carrier blocking layer, and at least one electrical contact layer.3. The method of claim 1, wherein the indium containing laser core isfabricated at a first temperature, and the subsequent layers are grownor deposited on the laser core at or below a second temperature that islower than the first temperature, to inhibit degradation of the lasercore.
 4. The method of claim 3, wherein the crystalline transparentconductive oxide layer is grown from an aqueous solution,
 5. The methodof claim 4, wherein the crystalline transparent conducting oxide iscombined with an amorphous or polycrystalline transparent conductingoxide layer deposited by physical vapor deposition.
 6. The method ofclaim 1, wherein the subsequent layers fabricated on the laser corecomprise at least one transparent and electrically conducting layerformed on or above an indium-containing active region as a waveguide orcladding layer and to provide electrical contact to theindium-containing active region.
 7. The method of claim 1, wherein thelaser core generates green light having a laser wavelength longer than515 nm.
 8. An (Al,Ga,In)N laser diode fabricated using the method ofclaim
 1. 9. A method of fabricating an (Al,Ga,In)N laser diode,comprising fabricating an indium containing laser core; and fabricatingsubsequent layers on the laser core, in a manner that inhibitsdegradation of the laser core, wherein the subsequent layers fabricatedon the laser core comprise a crystalline transparent conducting oxidegrown separately from the laser core and then bonded to the laser core.10. The method of claim 9, wherein the subsequent layers comprise one ormore of: at least one waveguide core layer, at least one waveguidecladding layer, at least one carrier injection layer, at least onecarrier blocking layer, and at least one electrical contact layer. 11.The method of claim 9, wherein the indium containing laser core isfabricated at a first temperature, and the subsequent layers are grownseparately from the laser core and then bonded to the laser core at orbelow a second temperature that is lower than the first temperature, toinhibit degradation of the laser core.
 12. The method of claim 9,wherein the subsequent layers fabricated on the laser core comprise atleast one transparent and electrically conducting layer formed on orabove an indium-containing active region as a waveguide or claddinglayer and to provide electrical contact to the indium-containing activeregion.
 13. The method of claim 12, wherein the transparent andelectrically conducting layer comprises one or more oxide layers orbonded (Al,Ga,In)N layers, and the transparent and electricallyconducting layer is deposited by a method other than crystal growth at atemperature greater than 550° C.
 14. The method of claim 9, wherein thelaser core generates green light having a laser wavelength longer than515 nm.
 15. An (Al,Ga,In)N laser diode fabricated using the method ofclaim
 9. 16. A method of fabricating an (Al,Ga,In)N laser diode,comprising fabricating an indium containing laser core; and fabricatingsubsequent layers on the laser core, in a manner that inhibitsdegradation of the laser core, wherein the subsequent layers fabricatedon the laser core comprise crystalline (Al,Ga,In)N layers grown on aseparate substrate and then bonded to the laser core.
 17. The method ofclaim 16, wherein the subsequent layers comprise one or more of: atleast one waveguide core layer, at least one waveguide cladding layer,at least one carrier injection layer, at least one carrier blockinglayer, and at least one electrical contact layer.
 18. The method ofclaim 16, wherein the indium containing laser core is fabricated at afirst temperature, and the subsequent layers are grown on the separatesubstrate and then bonded to the laser core at or below a secondtemperature that is lower than the first temperature, to inhibitdegradation of the laser core.
 19. The method of claim 16, wherein thesubsequent layers fabricated on the laser core comprise at least onetransparent and electrically conducting layer formed on or above anindium-containing active region as a waveguide or cladding layer and toprovide electrical contact to the indium-containing active region. 20.The method of claim 19, wherein the transparent and electricallyconducting layer comprises one or more oxide layers or bonded(Al,Ga,In)N layers, and the transparent and electrically conductinglayer is deposited by a method other than crystal growth at atemperature greater than 550° C.
 21. The method of claim 1, wherein thelaser core generates green light having a laser wavelength longer than515 nm.
 22. An (Al,Ga,In)N laser diode fabricated using the method ofclaim 16.